TWENTYNINE PALMS, Calif. — Kilo Company of the Third Battalion, Second Marines was deep in a simulated firefight on a recent morning here in the high California desert.

A group of Marines on the aptly named Machine Gun Hill unleashed round after round of live fire as another group of Marines tried to seize the surrounding valley from an imaginary enemy.

“They’re dug in,” shouted Brian Somers, the chief warrant officer of Kilo Company, describing the “enemy” forces, supposedly supported by a real state with real resources and who were theoretically returning fire. “This is conventional warfare.”

After 15 years of fighting terrorists, the United States military is learning how to fight big armies again.

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Marines cleaned expended rounds of ammunition this month during an exercise in California. American forces are adapting their training methods to fight larger enemy forces.CreditIvan Kashinsky for The New York Times

From the Middle East to South Asia to Africa, American forces for the past decade and a half have fought counterinsurgency and counterterrorist campaigns — essentially smaller-scale guerrilla warfare — rather than the large land wars of the past. But Russia’s invasion of Crimea, a surging China and an unpredictable North Korea have led American military commanders to make sure soldiers, sailors, airmen and Marines are trained in conventional warfare.

It is part of learning how to fight what the Pentagon calls the hybrid wars of the future, envisioned as a mix of conventional battles, insurgencies and cyberthreats.

“You’re looking at different level of capabilities when you’re talking about a higher-end threat, and the United States Army hasn’t fought against that type of enemy in a long time,” Gen. Mark A. Milley, the Army chief of staff, said in an interview. “The way we train won’t be the same because the environment now is totally different.”

Future wars, he said, “could have conventional forces, Special Forces, guerrillas, terrorists, criminals all mixed together in a highly complex terrain environment, with potentially high densities of civilians.”

In recent months the Army has held training exercises with hundreds of troops, tanks, drones, missiles and armored vehicles clashing against a supposed enemy force with similar abilities. The exercises are far different from what the United States faced in Afghanistan in 2001, when Qaeda insurgents and Taliban fighters disappeared into the hills and mountains and pulled the American military into a counterinsurgency guerrilla campaign that continues to this day.

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Marines this month at Twentynine Palms, Calif. After 15 years of fighting terrorists, the military is learning how to fight large land wars again. “The way we train won’t be the same because the environment now is totally different,” Gen. Mark A. Milley said.CreditIvan Kashinsky for The New York Times

Iraq, which began in 2003 as a classic movement of state-versus-state warfare with tanks and a big invading force, evolved into another counterinsurgency campaign. Today American forces are fighting yet another counterinsurgency against the Islamic State in Iraq and Syria.

“All of us, from the Army to the Navy to the Marine Corps, we fought well and courageously for 15 years against a tough foe,” said Gen. Robert B. Neller, the commandant of the Marine Corps. “But now we think about who might the next fight be.”

It will probably be, he said, “somebody who’s got electronic warfare, armored vehicles and the ability to maneuver.”

On the recent morning at Twentynine Palms, General Neller was joined by Adm. John M. Richardson, the chief of naval operations, to watch the training exercise — the first time a Navy chief had traveled to the Marine base to do so. Defense officials say the changing nature of war calls for closer cooperation between the services.

“When you look at this return of great power competition, one of the things that we have to pay more attention to, think harder about, is not only power projection, which is what we’ve been doing, but also sea control,” Admiral Richardson said. “Naval combat at sea. Work our way, fight our way in, from further out in the ocean.”

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Exercises in California this month teaching what the Pentagon refers to as hybrid warfare.CreditIvan Kashinsky for The New York Times

After the morning’s live-fire mortar pounding from Machine Gun Hill, the two chiefs drove 30 minutes down the road to observe another exercise, the retaking of an occupied urban area. Marines had built a small city in the desert, complete with advertising for cellphone companies on the sides of empty apartment buildings.

The town in the simulation looked straight out of Iraq, and could have been Falluja or Ramadi, except without people. The buildings were sand-colored, with a distinctive Iraqi look to them.

“What part of the world are we playing in?” General Neller asked. “Are the role players speaking English?”

The answer came back: “Arabic, sir.”

The interchange and the exercise showed that as American forces learn conventional warfare, they continue to train in counterinsurgency tactics to take on the Islamic State, Al Qaeda, the Taliban — or some other group.

“We should not expect the Chinese and Russians to forgo counterinsurgency warfare,” said Kathleen H. Hicks, the director of the international security program at the Center for Strategic and International Studies. Or, she said, “they may mass and stand and fight.”

A version of this article appears in print on , on Page A10 of the New York edition with the headline: Wars of the Future? Picture Big Armies and Many Fronts. Order Reprints | Today’s Paper | Subscribe